Showing papers on "Nafion published in 2002"

Micromechanics of actuation of ionic polymer-metal composites

Abstract: Ionic polymer-metal composites (IPMCs) consist of a polyelectrolyte membrane (usually, Nafion or Flemion) plated on both faces by a noble metal, and is neutralized with certain counter ions that balance the electrical charge of the anions covalently fixed to the backbone membrane. In the hydrated state (or in the presence of other suitable solvents), the composite is a soft actuator and sensor. Its coupled electrical-chemical-mechanical response depends on: (1) the chemical composition and structure of the backbone ionic polymer; (2) the morphology of the metal electrodes; (3) the nature of the cations; and (4) the level of hydration (solvent saturation). A systematic experimental evaluation of the mechanical response of both metal-plated and bare Nafion and Flemion in various cation forms and various water saturation levels has been performed in the author’s laboratories at the University of California, San Diego. By examining the measured stiffness of the Nafion-based composites and the corresponding bare Nafion, under a variety of conditions, I have sought to develop relations between internal forces and the resulting stiffness and deformation of this class of IPMCs. Based on these and through a comparative study of the effects of various cations on the material’s stiffness and response, I have attempted to identify potential micromechanisms responsible for the observed electromechanical behavior of these materials, model them, and compare the model results with experimental data. A summary of these developments is given in the present work. First, a micromechanical model for the calculation of the Young modulus of the bare Nafion or Flemion in various ion forms and water saturation levels is given. Second, the bare-polymer model is modified to include the effect of the metal plating, and the results are applied to calculate the stiffness of the corresponding IPMCs, as a function of the solvent uptake. Finally, guided by the stiffness modeling and data, the actuation of the Nafion-based IPMCs is micromechanically modeled. Examples of the model results are presented and compared with the measured data.

Ionic Conductivity of an Extruded Nafion 1100 EW Series of Membranes

Abstract: The proton conductivity of a series of extruded Nafion membranes @of equivalent weight ~EW! of 1100 and nominal dry thickness of 51, 89, 127, and 178 mm# has been studied. Measurements were made in 1 M H2SO4 at 298 K using a four-electrode, dc technique. The membrane area resistance increases with thickness, as expected, from 0.07 to 0.16 V cm2 for Nafion 112 and Nafion 117, respectively. However, in contrast to the published literature, after correcting for the membrane thickness, the conductivity of the membranes decreases with decreasing membrane thickness. For example, values of 0.083 and 0.16 S cm21 were obtained for Nafion 112 and 117 membranes, respectively. In situ current-interrupt measurements in a proton exchange membrane fuel cell confirmed the relatively poor conductivity of the membrane electrode assemblies ~MEAs! based on the thinner membranes. While a high contact resistance to the electrodes may have contributed to the in situ MEA resistance, water balance measurements over the MEA showed that the high resistance was not due to a low water content or to an uneven water distribution in the MEAs. The implications of the findings for the understanding of the membrane properties are discussed.

Evidence of elongated polymeric aggregates in Nafion

Abstract: Small-angle X-ray and neutron-scattering techniques have been used to probe the structure of swollen Nafion membranes in the range 10−10000 A. From analyzing the scattering data as a function of the polymer volume fraction and using a contrast variation method for the neutron experiments, we suggest a new structural model of Nafion in the hydrated state. It is based on the aggregation of the ionomer chains into elongated polymeric bundles with a diameter on the order of 40 A and a length larger than 1000 A, surrounded by the electrolyte solution.

Silicon Oxide Nafion Composite Membranes for Proton-Exchange Membrane Fuel Cell Operation at 80-140°C

Abstract: Silicon oxide/Nafion composite membranes were studied for operation in hydrogen/oxygen proton-exchange membrane fuel cells (PEMFCs) from 80 to 140°C. The composite membranes were prepared either by an impregnation of Nafion 115 via sol-gel processing of tetraethoxysilane or by preparing a recast film, using solubilized Nafion 115 and a silicon oxide polymer/gel Tetraethoxysilane, when reacted with water in an acidic medium, undergoes polymerization to form a mixture of SiO 2 and siloxane polymer with product hydroxide and ethoxide groups. This material is referred to as SiO s /-OH/-OEt. When Nafion is used as the acidic medium, the SiO 2 /siloxane polymer forms within the membrane. All composite membranes had a silicon oxide content of less than or equal to 10 wt %. The silicon oxide improved the water retention of the composite membranes, increasing proton conductivity at elevated temperatures Attenuated total reflectance-Fourier transform infrared spectroscopy and scanning electron microscopy experiments indicated an evenly distributed siloxane polymer of SiO 2 /-OH/-OEt in the composite membranes. At a potential of 0.4 V, silicon oxide/Nafion 115 composite membranes delivered four times the current density obtained with unmodified Nafion 115 in a H 2 /O 2 PEMFC at 130°C and a pressure of 3 atm. Furthermore, silicon oxide-modified membranes were more robust than the control membranes (unmodified Nafion 115 and recast Nafion), which degraded after high operation temperature and thermal cycling.

Novel sulfonated polyimides as polyelectrolytes for fuel cell application. 2. Synthesis and proton conductivity of polyimides from 9,9-bis(4-aminophenyl)fluorene-2,7-disulfonic acid

Abstract: A new sulfonated diamine monomer, 9,9-bis(4-aminophenyl)fluorene-2,7-disulfonic acid (BAPFDS), was synthesized by direct sulfonation of the parent diamine, 9,9-bis(4-aminophenyl)fluorene (BAPF), using fuming sulfuric acid as the sulfonating reagent. A series of sulfonated polyimides with different sulfonation degrees were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), BAPFDS, and common nonsulfonated diamines. The resulting sulfonated polyimides generally showed good solubility in m-cresol and DMSO. Proton conductivities of these polyimide membranes were measured as the functions of relative humidity and temperature. The resulting homopolyimide, NTDA-BAPFDS, displayed proton conductivities quite similar to those of Nafion 117 in the whole humidity range (RH < 100%). At 100% relative humidity, all the BAPFDS-based polyimide membranes showed proton conductivities similar to or higher than those of Nafion 117. In addition, BAPFDS-based polyimide membranes exhibited much better water sta...

Single Wall Carbon Nanotube−Nafion Composite Actuators

Abstract: Single wall carbon nanotube (SWNT)−Nafion composite actuators have been demonstrated for 0.1−18% w/w doping of purified SWNTs within the polymer matrix. The high purity SWNTs, >95% w/w, were homogeneously dispersed into the polymer through a series of homogenization and high sheer mixing techniques. The efficient distribution of the high aspect ratio, conductive SWNT materials allowed for the actuation of the Nafion membrane to be reached at SWNT doping levels as low as 0.5% w/w. Utilizing a two-electrode bimorph cantilever actuator immersed into an aqueous lithium chloride solution, tip deflections up to 4.5 mm were observed.

Effect of Ammonia as Potential Fuel Impurity on Proton Exchange Membrane Fuel Cell Performance

Abstract: Effects of NH 3 on proton exchange membrane fuel cell performance are reported. Traces of NH 3 in the anode feedstream cause a decrease in cell current. The extent of the effect depends on NH 3 concentration and time of exposure of the anode to NH 3 . High trace levels and long time of exposure result in severe and irreversible disability. We discuss possible mechanisms by which NH 3 affects cell performance. A method for trapping ammonia from contaminated fuel streams using a H + form ion exchange resin is presented.

A new insight into Nafion structure

Abstract: The structure of hydrated Nafion membranes neutralized by N(CH3)4+ ions has been investigated by the small angle neutron scattering technique. First, the spectra show that the counterions are condensed at the interface between hydrophilic and hydrophobic phases. More importantly, by applying a contrast variation method, the normalized scattered intensity of membranes swollen in several D2O/H2O mixtures was analyzed. The N(CH3)4+ counterions were used as a probe to identify the nature of the scattering entity that is polymeric aggregates surrounded by ionic groups and water molecules, contrary to the generally accepted model. This new insight into the Nafion structure will allow one to reconsider the swelling process and the degree of mesoscopic orientation in such perfluorinated systems.

Ion and Water Transport Characteristics of Perfluorosulfonated Ionomer Membranes with H+ and Alkali Metal Cations

Abstract: Ion and water transport characteristics of Nafion ionomer membranes were investigated systematically in the mixed cation form of H+ and various kinds of alkali metal cation systems, which were prepared by equilibrating the membranes in the mixtures of HCl and alkali chloride in aqueous solutions of various mixing ratios. The membrane cationic composition showed that cations of larger atomic number had a higher affinity to sulfonic acid groups but less water content in the membrane than those of smaller atomic numbers. The net ionic conductivity was decreased, in any case, by the presence of alkali metal cations in the membrane. Different kinds of the interaction mode among cations were observed between H/Li or H/Na systems and H/K, H/Rb, or H/Cs systems. The interaction between alkali metal cations appeared to increase as the atomic number of the alkali metal cation increased. The water transference coefficient (electro-osmosis drag coefficient) increased from 2.5 to more than 10 by the presence of alkali...

The nature of proton transport in fully hydrated Nafion

Abstract: The diffusion of protons has been studied in fully hydrated Nafion® with a recently constructed non-equilibrium statistical mechanical transport model. Radial cross-sectional profiles of the effective friction and diffusion coefficients were computed in an electrolyte membrane pore with a hydration of 22.5 water molecules per sulfonic acid fixed site. Input parameters were taken from recent SAXS measurements of the hydrated membrane and electronic structure calculations of water clusters with CF3SO3H, the associated acid for the side chain termination. The calculations revealed that the effective friction coefficient increases by more than two orders of magnitude as the proton is brought from the center of the pore to within 4 A of the fixed sites. The model calculated a diffusion coefficient of 1.92 × 10−9 m2 s−1, without ‘fitting’ any parameters, for a proton moving along the pore center, in good agreement with experimental measurements. In addition, the model also identified a predominantly vehicular transport mechanism in regions of the cross section of the pore where the proton is within 12 A of the pore wall. This was distinguished from the central region of the pore (within 4 A of the center axis) where a component of the conduction is via the Grotthuss mechanism. This investigation has demonstrated the applicability of this transport model in the prediction of diffusion coefficients in fully hydrated membranes.

Nafion-bifunctional silica composite proton conductive membranes

Abstract: Sol–gel derived sulfonated phenethylsilica with hydrophilic –Si-OH and proton conductive –SO3H functional groups was used as a bifunctional additive to improve the proton conductivity and water uptake characteristics of Nafion. Nafion-bifunctional silica (NBS) composite membranes were prepared by casting Nafion–ethanol solutions mixed with sulfonated phenethylsilica sol. The ion exchange capability of NBS composite membranes increases linearly with the amount of bifunctional silica incorporated, and is about 1.9 × 10−3 mol SO3H g−1 for NBS with 7.5 wt% silica. Liquid water uptake measurements showed that NBS composite membranes have higher water uptake (g H2O g−1 composite membrane) than bare recast Nafion while the degree of hydration (i.e., nH2O–SO3H) remains fairly constant. The NBS composite membranes showed improved proton conductivity when compared with bare recast Nafion and Nafion 117 membranes at 80 °C and over a range of relative humidity.

Optimal environment for glucose oxidase in perfluorosulfonated ionomer membranes: improvement of first-generation biosensors.

Abstract: An optimal environment for glucose oxidase (GOx) in Nafion membranes is achieved using an advanced immobilization protocol based on a nonaqueous immobilization route Exposure of glucose oxidase to water−organic mixtures with a high (85−95%) content of the organic solvent resulted in stabilization of the enzyme by a membrane-forming polyelectrolyte Such an optimal environment leads to the highest enzyme specific activity in the resulting membrane, as desired for optimal use of the expensive oxidases Casting solution containing glucose oxidase and Nafion is completely stable over 5 days in a refrigerator, providing almost absolute reproducibility of GOx−Nafion membranes A glucose biosensor was prepared by casting the GOx−Nafion membranes over Prussian Blue-modified glassy carbon disk electrodes The biosensor operated in the FIA mode allows the detection of glucose down to the 01 μM level, along with high sensitivity (005 A M-1 cm-2), which is only 10 times lower than the sensitivity of the hydrogen p

Stress-Strain Curves of Nafion Membranes in Acid and Salt Forms

Abstract: Nafion membranes have been studied using dynamic mechanical analysis in the controlled force mode and clamp of tension film measurements for as received commercial Nafion membranes with different solvent contents and with different cations (Li+, Na+, K+, Rb+ and Cs+). In comparison with as-received Nafion membranes, the initial slope of the stress-strain curves decreases with increasing water contents in the Nafion membrane and also with the addition of methanol, ethanol and ethanol/water (50/50%, v/v) mixture. The initial slope decreases also with increasing temperatures. On the other hand, the initial slope increases when the cation is replaced, in the following order: Li+, Na+, K+, Cs+ and Rb+. Nafion in salt form normally shows an increase in the initial slope from room temperature up to approximately 90oC, after which the slope decreases with increasing temperatures.

Transport and Structural Studies of Sulfonated Styrene-Ethylene Copolymer Membranes

Abstract: The structure and proton conductivity of sulfonated styrene-ethylene copolymers have been studied. Conductivities in excess of 0.1 S/cm are obtained depending upon copolymer composition and sulfonation level. The dependence of conductivity on humidity has been measured and compared with that of Nafion and a partially sulfonated block copolymer. X-ray and neutron scattering studies suggest the presence of a bicontinuous network of hydrophilic and hydrophobic domains in water-swollen samples.

Carboxylated and Sulfonated Poly(arylene-co-arylene sulfone)s: Thermostable Polyelectrolytes for Fuel Cell Applications

Abstract: The synthesis of novel soluble copolyarylenes, their functionalization with sulfonic and carboxylic acid groups, and the determination of fuel cell relevant parameters (swelling behavior, methanol permeation, and ionic conductivity) are described. The Ni(0)-catalyzed homocoupling reaction of aryl chlorides was employed for the polymerizations. Carboxylic acid groups were incorporated by copolymerization of methyl 2,5-dichlorobenzoate and subsequent hydrolysis. The composition was varied from 53 to 100 mol % carboxylic acid groups. Sulfonic acid groups were introduced by sulfonation with chlorosulfonic acid. Flexible and transparent membranes with sulfonic and/or carboxylic acid groups were prepared that exhibited higher proton conductivities (values in the range of σ = 0.11−0.23 S/cm) compared to those of Nafion and sulfonated PEEK as a result of higher ion exchange capacity and water content. Incorporation of carboxylic acid groups led to a reduced water uptake but lower conductivities.